These
maps, both compiled by Bill Fields, show retreat of the Muir Glacier from 1880
until the 1980s. The record has subsequently been extended by satellite measurements.
Even though the Muir has been shrinking for over 200 years, it is not a good indication that
the Earth's climate is warming. A tidewater glacier, it's rate of growth or recession is
determined by the calving of icerbergs at the terminus. A nearby tidewater glacier, the
Great Pacific, has been growing rapidly.

(Top map from the American Geographic Society Collection
archived at the National Snow and Ice Data Center, University of
Colorado at Boulder. Bottom map Hall et. al., 1995)

by John Weier - April 14, 1999

Since well before global warming became a heated political
issue, scientists have been trying to determine the rate at which our
planets temperature is increasing. While placing many thermometers around
the world would appear to be the solution, local temperatures can vary widely
across regions and from one year to the next. Instead, researchers have found
they can obtain a measure of average global temperatures by using
satellites to monitor heat-sensitive objects on the ground. Of these objects,
glaciers are among the most reliable indicators of climate change.

Alpine glaciers, like this one near Mt. McKinley, Alaska, change
in response to the local climate. By monitoring the change in size of glaciers around the world, scientists can learn
about global climate change. (Photograph by Klaus J.
Bayr, Keene State College, 1990)

One
method of measuring glaciers is to send researchers onto the ice with
surveying equipment. The Muir Glacier, shown here around 1950, has been studied for
over 200 years. (Photograph from the American Geographic Society Collection
archived at the National Snow and Ice Data Center, University of
Colorado at Boulder)

Despite typical glaciers massive sizes, monitoring them is not always
an easy task. Only specific types of small glaciers are good measures of
climate change. Some glaciers are too large to measure accurately, and others
are simply too unpredictable. Once scientists find a suitable glacier, they
must take satellite images of the ice for a minimum of five years and compare
the results. They then have to look closely at the outside edge of the glacier
(the glaciers terminus). If a large
percentage of the glaciers edge
is receding then the area around the ice is growing warmer, and if a large
percentage is expanding then the area is growing cooler. When enough
measurements from many different parts of the world have been gathered, the
researchers can determine whether the earth is growing warmer or cooler.

The terminus of the Pasterze
glacier, Austria. It dwarfs the three hikers at lower right. (Photograph by Klaus J.
Bayr, Keene State College, 1988)

Types of Glaciers

Glaciers form when snow accumulates on a patch of land over
tens to hundreds of years. The snow eventually becomes so thick that it
collapses under its own weight and forms dense glacial ice. When enough of the
ice is compacted together it succumbs to gravity and begins to flow downhill or
spread out across flat lands (Williams and Hall, 1993).
There are many
different types of glaciers, and not all of them are good indicators of climate change.
"Glaciers that do tend to be good climate indicators are small land-based,
non-surge type glaciers. They respond directly to both regional temperature and
precipitation [snow]," said Dorothy Hall, a hydrospheric scientist at
NASAs Goddard Flight Space Center. She and a team of scientists from
around the globe have used satellites over the past 25 years to measure changes
in glaciers in Europe, Iceland and Alaska.

More than 90 percent of the 33 million cubic kilometers of
glacier ice in the world is locked up in the gigantic Greenland and Antarctic ice sheets. Because
they are so massive and exist in such frigid latitudes, large-scale changes
are very difficult to track and verify, said Hall.

In addition to the ice sheets, there are two types of small glaciers that
make for bad climate gauges. "Both surge glaciers and tidewater glaciers
have their own cycles of advance and retreat. These cycles are certainly
related to climate, but we are not exactly sure how," said Hall. Even if
the climate changes in the region, these glaciers would most likely maintain
their distinctive patterns of behavior.

Tidewater
glaciers terminate in the ocean, where the boundary can be marked by awesome ice cliffs 200 feet high. When huge blocks of
ice break off to form icebergs (a process known as calving) the waves can be large enough to capsize ships. (Photograph by Bill Field, 1971)

Surge glaciers can sluice down a
valley at rates of up to a few kilometers a day, said Hall.
Once they attain their final destination, they stagnate or gradually retreat for
the next ten-to-fifty years. Tidewater glaciers, on the other hand, advance for
roughly a thousand years before reaching their destination, explained Hall.
When they encounter the sea, they calve and drop icebergs into the water as they
continue to make their way out into the ocean. They then pull back and retreat
over the course of one or two hundred years.

The Muir tidewater glacier
in Alaskas Glacier Bay has been observed for nearly 200 years. Explorers
in the mid-1700s recorded observations of Muir at its peak. They noticed that
about two hundred years ago it began to recede. Recent measurements show that
the glacier has withdrawn more than 90 kilometers (Hall et. al., 1994).

This
map, compiled by the glaciologist Bill Field, shows the changes in the glaciers of Muir Inlet, Alaska, from 1880 to 1941.
Large map. (Map from the American Geographic Society Collection
archived at NSIDC)

Challenges to detecting glacial change

Most small glaciers in the world are not as exciting as surge and tidewater
glaciers. They may form on the tops of mountains and slowly make their way
over hundreds of years through valleys and
across plains. A change in the yearly temperature around a glacier can cause it
to expand or contract. Hall said, "By measuring the terminus of the
glacier periodically, scientists can tell if the local climate is changing. To
get an indication of whether the global climate is changing, researchers must
monitor these small glaciers across the planet, and for many years to decades"

Before man-made satellites were invented, monitoring enough glaciers to get a
measure of global climate change would have been impossible, said Hall.
Some glaciers tend to extend in many different directions. As they melt, one part
may retract and another part may stay put. Between 1973 and 1987, many outlet glaciers
of the Vatnajökull ice cap in Iceland has been steadily receding.
Yet, parts of the glacier haven't moved at
all (Hall et al. 1992). If scientists were to travel to the glacier and
measure just this section, they could be deceived. Measurements have to be made
regularly at every extension of the ice cap to see if the glacier as a whole is
melting, Hall said. While this may be feasible for one glacier, it is nearly
impossible to measure a hundred glaciers this way.

Satellite sensors
such as those on Landsat 5 and Landsat 7 allow scientists to measure the entire
rim of any glacier on an annual basis, cloud cover permitting. These satellites each have seven
different types of light detectors (photoreceptors) on board, which acquire
images of different wavelengths of sunlight being reflected off of the Earth.
One light detector records only the blue light coming off the Earth (band 1).
Another observes all the yellow-green light (band 2) and still another picks up
on all the near-infrared light (band 4). The satellites move in circular
orbits, very nearly from pole-to-pole, around the Earth and scan strip after
strip of our spinning planet. The satellites images are then beamed back
to the surface, where Hall and other researchers can examine them.

The trick to measuring the extent of a glacier using a satellite image
lies in distinguishing the glaciers edge from the surrounding land.
Unfortunately, this task isnt as simple as drawing a line on the image
between what looks like a glacier and what looks like land. Both glaciers and
the surrounding ground often have the same dark gray coloring and can be easily
confused by sight.

This aerial photograph
shows one of the problems inherent in studying glaciers with remote sensing. The dark
bands on the surface of the glacier, caused by ground-up rocks, make it difficult to distinguish
the glacier's edges from the surrounding terrain.

To separate glacier from surrounding land more
accurately, scientists must look at specific types of light being reflected. As
can be seen through a prism, sunlight contains many different individual colors
(wavelengths). When sunlight strikes objects, certain colors of the spectrum
are absorbed and others are reflected. The reflected wavelengths give an object
its color.

Like most white objects, the glacier reflects nearly all the
colors of the visible spectrum, including the yellow-green sunlight. Yet,
a glacier absorbs near-infrared wavelengths of solar energy (light to the right of red
on the color spectrum). The research teams using the Landsat 5 and Landsat 7
satellites differentiate snow from other solid materials by looking at the
difference between the infrared and yellow-green wavelengths that are reflected.
Whenever the difference is large, the area in question is likely to be snow or
glacier ice (Hall et al. 1998). By tracking the edges of a glacier from year to
year, scientists are often able to see if it is receding or advancing.

Hall
explained, while satellite data are easier to collect than ground
measurements, scientists still have to record images for many years before they
can be certain a glacier is changing, and it is critical to abtain ground measurements to corroborate
results deduced from analysis of satellite data. The most obvious reason for the
scientists uncertainty has to do with the "resolution" of the
satellite images. The Landsat 5 and Landsat 7 satellites have a resolution of
30 meters, which means that each pixel (or picture element) on the image
represents a 30-by-30-meter patch of land. "Though the glacier will
respond immediately to the changes in climate, it may take five to twenty years
before we can see the changes in the glacier from satellites," said
Hall.

This pair of images from the Landsat
series of satellites, taken more than a decade apart, shows the continuing retreat of the Muir glacier.
The front face of the glacier is marked by the arrow in each image, and moved more than 7km between 1973 and 1986.
(Image courtesy Dorothy Hall, NASA Goddard Space Flight Center)

Implications of glacial shrinkage

In some cases,
a glacier that recedes around the edges may be growing thicker in areas near the
center. Since visible satellite sensors cannot penetrate the surface of the
glacier, scientists may be led to believe, at least in the short term, that the
glaciers are losing mass. Thus, a glacier must be measured for many years to detect
sustained changes in its edge or "ice front."

"It is very hard to measure volume changes in
glaciers. People are required to put stakes in the glacier and come back a year
later to see how much of a change there is in the height of the glaciers," said
Hall. She said that this is a very labor-intensive activity and is done on very
few glaciers in the world. The only way to make sure the glacier is pulling
back due to a loss of mass is to study it over a period of many years.

On average, scientists are finding that glaciers across the globe are steadily
shrinking, said Hall. These findings confirm suspicions that the world is
heating up. Researchers generally believe the warming trend may be the result
of natural, cyclical changes of the Earths climate, and possibly in combination
with effects of the large-scale burning of fossil fuels by humans since
the industrial revolution. However, the cause of global warming is currently unknown.

The last time the Earth warmed extensively,
120,000 years ago, the Greenland ice sheet drained into the ocean and the sea
rose roughly 20 feet above where it is now (Williams and Hall, 1993). Such an
increase today would flood coastal communities and low-lying
countries such as Holland and Bangladesh, as well as much of the U.S.
state of Florida.

This scientist is
setting up instruments to observe a glacier. Because glaciers are so large, remote, and widespread,
it is impossible to measure them all from the ground, or even from aircraft. Satellites such as
Landsat-7 are used in addition. (Photograph courtesy Dorothy Hall, NASA Goddard Space Flight Center)